WO 00 65538 discloses a differential image generated in which the contour of the object whose position is to be detected appears only if the object moves. In this method, only moving objects are automatically recorded in a scene, which makes it possible for images to be evaluated simply and reliably. A CCD or CMOS camera which is equipped with a wide-angle lens and mounted as high as possible within the interior of the vehicle, preferably in the roof lining of the vehicle, is used for recording images.
The camera or a different optical sensor system can be arranged between the front seats; with its viewing angle it then covers the area between the dashboard and the seat backrest and between the roof lining and the seat cushion.
The known method is used in order reliably to determine information about the position of the front passenger, in particular about the position of the front passenger's head, within the interior of the vehicle, and to use this information for releasing an airbag system.
On the other hand, it is also known for a door contact of the vehicle to be read out in order for this information to be evaluated for monitoring the airbag system. A monitoring device classifies the occupancy of a vehicle seat, in particular of the passenger seat, into various possible occupancy classes (empty seat; child-seat facing backwards; child-seat facing forwards; adult person) and permits, by means of defined transitional probabilities, transitions between these occupancy classes. Provided that the vehicle door is open, the airbag system should not, however, be enabled. For that reason, a door contact, for example a mechanical door contact, of the vehicle generates a signal which is fed to the monitoring system. To achieve this, an electric line exists from the door contact to the camera or sensor system in the roof lining.
A cable has, however, the disadvantage that laying it entails a cost. A long cable brings with it additional EMC problems (EMC=electromotive compatibility). The usually cheap door contact of the vehicle becomes a safety-relevant part because it affects the releasing of the airbag.